The invention is directed to a process for obtaining pure L-leucine from an aminoacid (aminocarboxylic acid) mixture which contains at least 45 weight percent L-leucine, at most 40 weight percent L-isoleucine and at most 25 weight percent of other aminoacids, in each case based on the dry material.
L-leucine is employed as a pharmaceutical, e.g. as a component of infusion solutions based on aminoacids. Until now the industrial manner of obtaining L-leucine is the so-called extraction process. For this purpose proteins are hydrolyzed to aminoacid mixtures and an industrial L-leucine is obtained from this by fractional crystallization and/or a chromatographic process (ion exchange, ion exclusion and/or the molecular sieve effect).
The thus obtained technical leucine contains in addition to inorganic salts such as sodium chloride, sodium sulfate, ammonium chloride or ammonium sulfate other aminoacids, especially L-isoleucine. To produce L-leucine in pharmaceutical quality there must be removed these impurities, whereby especially the separation of L-isoleucine causes difficulties since isoleucine has the same empirical formula C.sub.6 H.sub.13 NO.sub.2 as leucine and the two aminoacids differ only in the structure of the aliphatic side chain. ##STR1##
Based on the very similar structural characteristics these aminoacids exhibit very similar properties in their physical and chemical behavior. This is also the reason why commercial L-leucine frequently contains significant amounts of L-isoleucine in addition to other aminoacids (e.g. L-valine, frequently also L-methionine). In regard to these conditions see also Richard J. Block [Arch. Biochem. Vol. 11, 501 (1946), page 512].
The difficult purification of industrial L-leucine has already been realized via the copper complex. In a similar process the cobalt complexes of the aminoacids were also separated by extraction with alcohol. However, in this process there is the problem of the recovery of the metal and the further purification of the L-leucine.
Other authors have described the precipitation of leucine with aromatic sulfonic acids. Thus there has been proposed the use of 2-bromotoluene-5-sulfonic acid or naphthalene-2-sulfonic acid for the precipitation of L-leucine. Also benzenesulfonic acid and p-toluenesulfonic acid have been employed for obtaining L-leucine starting from industrial L-leucine. In this process the precipitate must be purified by numerous recrystallizations and the separation of the frequently very toxic precipitation agent causes an additional problem.
Furthermore fractions of the acid protein hydrolysates rich in L-leucine have been purified by establishing an L-isoleucine concentration of about 1.5% with addition of water at a specific pH and after ascertaining the methionine content oxidizing the methionine by addition of hydrogen peroxide. Clarification with activated carbon is carried out, adjustment of the pH to 1.0 to 1.5, cooling and separation of a crude L-leucine. This is dissolved again at pH 0.5 and again purified by precipitation at pH 1.0 to 2.0. This process is repeated frequently until the desired purity of L-leucine is attained. The process which is described in European patent 14867 requires working with very dilute materials and besides the process requires so many steps that it scarcely can be considered for use on an industrial scale.
Furthermore the known processes for the purification of industrial L-leucine to obtain pure L-leucine in pharmaceutical quality have a decisive fault. Nothing is said in regard to the purity of the enantiomer of the L-leucine. Leucine, however, is among the quickest racemizing aminoacids under the conditions of the customary acid hydrolysis of protein (Liebig's Ann. Chem 1981, pages 354 to 365).
The different biological effect of D-leucine and L-leucine is known so that a process which simultaneously permits the separation from D-leucine and from other impurities present in industrial L-leucine is very much desired.